I hate (or at least dislike): the weak connection of the bagman support, mine wiggles loose no matter what. It is not made to take the abuse of singletrack or dirt roads. Getting behind the saddle, although I seldom if ever need to do this. The thin and fiddly leather straps. finally. How floppy the bag is when empty without the bagman inside it.
I think something similar to pro_out's approach may be worth looking at:
http://www.bikepacking.net/forum/index.php/topic,605.msg14306.html#msg143061000 cu inches is ~16 litres, which is about medium dry bag sized. A dry bag is obviously not as convenient as a top-opening saddlebag, so maybe a similar rack could be fashioned which would support a traditional saddlebag.
Or perhaps what is needed is a saddlebag variation which goes back rather than sideways - to allow sliding back off the saddle - which is getting into the realms of lightweight rack-top bags. Perhaps an ultralight backpack design could be adapted. I noticed recently that Jandd will sell you both parts of a KilckFix system:
http://www.jandd.com/detail.asp?PRODUCT_ID=FKLICKFIXThis may help in designing or adapting something suitable.
Dr. Matt...